1. Field of the Invention
The present invention relates to non-aqueous electrolyte secondary batteries. More particularly, the invention relates to a non-aqueous electrolyte secondary battery comprising a positive electrode employing lithium cobalt oxide as the positive electrode active material, a negative electrode employing molybdenum dioxide as the negative electrode active material, and a non-aqueous electrolyte, and the non-aqueous electrolyte secondary battery is characterized in that it is capable of preventing the battery performance from degrading and of obtaining a sufficient battery capacity even when the battery is continuously charged with a very small current while being kept at a constant voltage for a long period of time.
2. Description of Related Art
In recent years, a non-aqueous electrolyte secondary battery, which employs a non-aqueous electrolyte solution and has a high electromotive force, has been widely used as a new type of secondary battery that achieves high power and high energy density.
This type of non-aqueous electrolyte secondary battery has been used as a power source for backing up memory data in mobile devices, in addition to use in a primary power source of mobile devices. In recent years, power sources for memory backup have been demanded to have higher energy densities as primary power sources in mobile devices tend to have higher energy densities.
The positive electrode active materials commonly used for non-aqueous electrolyte secondary batteries are lithium-transition metal composite oxides such as lithium cobalt oxide, lithium nickel oxide, and lithium manganese oxide having a spinel structure. The negative electrode active materials commonly used therefor include lithium metal, lithium alloys, carbon materials capable of intercalating and deintercalating lithium ions, lithium titanium oxides, and molybdenum oxides.
In recent years, a non-aqueous electrolyte secondary battery of the type described above has been proposed, which employs LiCoO2 or LiNiO2 as the positive electrode active material, and molybdenum oxide or iron sulfide as the negative electrode active material to prevent considerable deterioration in battery performance because of the heat used in reflow soldering (see Japanese Published Unexamined Patent Application No. 2000-243454, for example).
When the above-described non-aqueous electrolyte secondary battery is used as a power source for memory backup with a working voltage of about 3.0 V, it is necessary that the battery be charged with a very small current of about 1 μA to 5 μA while being kept at a constant voltage of about 3.0 V for a long period of time.
In the non-aqueous electrolyte secondary battery used as a power source for memory backup with a working voltage of about 3.0 V, a problem with using lithium manganese oxide having a spinel structure as the positive electrode active material is that because of its low specific capacity, the battery capacity cannot be high, and the storage performance is also poor. A problem with using LiCO1/3N1/3Mn1/3O2, LiNiO2, and the like as the positive electrode active material is that the working voltage becomes lower than that obtained with lithium cobalt oxide LiCoO2, so a sufficient capacity cannot be obtained when the battery is used with a working voltage of about 3.0 V.
In the case that the non-aqueous electrolyte secondary battery employing lithium cobalt oxide LiCoO2 as the positive electrode active material is used as a power source for memory backup with a working voltage of about 3.0 V, it is possible to use lithium titanium oxide Li4Ti5O12 or a molybdenum oxide, such as MoO2.5 and molybdenum dioxide MoO2, as the negative electrode active material.
A problem with using Li4Ti5O12 as the negative electrode active material is that the filling density is poorer than the molybdenum oxides, and a high battery capacity will not be attained. A problem with using a molybdenum oxide having a high oxidation number such as MoO2.5 is that the working voltage becomes higher than when using molybdenum dioxide MoO2, so that a sufficient capacity cannot be obtained.
For these reasons, in order to obtain a sufficient capacity when using a non-aqueous electrolyte secondary battery as a power source for memory backup with a working voltage of about 3.0 V, it is believed preferable that the positive electrode active material be lithium cobalt oxide LiCoO2 and the negative electrode active material be molybdenum dioxide MoO2.
Nevertheless, when the non-aqueous electrolyte secondary battery employing lithium cobalt oxide LiCoO2 as the positive electrode active material and molybdenum dioxide MoO2 as the negative electrode active material is used as a power source for memory backup with a working voltage of about 3.0 V, the battery performance considerably degrades.
The reason is as follows. In the case that a non-aqueous electrolyte secondary battery that has a positive electrode employing lithium cobalt oxide as the positive electrode active material and a negative electrode employing a carbon material as the negative electrode active material is used as the main power source of an ordinary portable device, the negative electrode is charged to about 0.1 V versus lithium metal during charge. Consequently, a surface film with good ionic conductivity forms on the surface of the carbon material of the negative electrode. This surface film keeps the negative electrode and the non-aqueous electrolyte solution from reacting with each other, making it possible to prevent decomposition of the non-aqueous electrolyte solution and destruction of the structure of the negative electrode.
In contrast, when the non-aqueous electrolyte secondary battery is used as a power source for memory backup with a working voltage of about 3.0 V, the positive electrode active material, lithium cobalt oxide, cannot yield a sufficient specific capacity unless it is charged at 4.0 V or higher versus lithium metal, and if the potential of lithium cobalt oxide becomes 4.2 V or higher versus lithium metal, decomposition of the non-aqueous electrolyte solution becomes considerable at the positive electrode. For this reason, the negative electrode can be charged only to about 1.0 V versus lithium metal, and the surface film will not be formed on the negative electrode surface.
Generally, a carbon material such as carbon black is added to the negative electrode as a conductive agent in order to enhance the conductivity in the negative electrode. However, as described above, because the surface film with good ionic conductivity does not form on the negative electrode surface, the carbon material used as the conductive agent in the negative electrode reacts with the non-aqueous electrolyte solution, causing decomposition of the non-aqueous electrolyte solution and destruction of the structure of the negative electrode. Thus, the battery performance is degraded.